Migration of seismic data is a computer-implemented data processing technique that uses a subsurface model of seismic wave propagation velocity to move subsurface reflection points to their true depth, thus providing an image of the subsurface. Reverse-time migration (RTM) is a very high fidelity imaging method which is commonly applied in complex geology settings. More accurate migration is performed in an iterative fashion, with the assumed velocity model being updated at each iteration cycle.
In complex geology, accurate velocity model building is critical for successful seismic imaging. This is particularly true in geological contexts dominated by high-contrast media such as salt. Such high-contrast media produce large-amplitude elastic conversions and scattering at their boundaries, and these elastic waves should not be ignored in imaging. By ignoring them, one means imaging them with an acoustic algorithm, which focuses only on longitudinal (P) waves, as opposed to an elastic algorithm, which focuses on both longitudinal and shear (S) waves. Elastic waves propagating in a rock medium cause local stretching (“strain”) of the rock, but the rock goes back to zero strain when the elastic wave passes. Shear waves must be present to demonstrate the elastic properties of the rock medium, and hence seismic waves that include shear waves are referred to as elastic waves. The reason shear waves must be included in imaging is that, firstly, scattering and/or reflection, even of (P) waves, from high-contrast boundaries show different amplitudes and sometimes different phases when the medium is treated as acoustic versus elastic. Secondly, ignoring them and treating them like noise will produce a large amount of noise in the imaged section. Thirdly, these elastic waves provide valuable information for updating the velocity model for further imaging iterations. Traditional imaging and model building with the acoustic RTM method ignore the elastic nature of the acquired seismic data in the real world. The acoustic-only algorithms will not merely fail to take advantage of this elastic scattering information in the data, but the present inventors have found that they may in fact lead to a false determination of the high-contrast geo-body boundary position.
Elastic RTM has rarely been used in real data processing, but references on the subject include Chang and McMechan, “3-D elastic prestack, reverse-time depth migration,” Geophysics 59, 597-609 (1994); for a general reference on elastic wave propagation, see Walley and Field, Encyclopedia of Materials: Science and Technology, Elsevier, chapter titled “Elastic Wave Propagation in Materials” (2005). These references are incorporated herein in all jurisdictions that allow it. However, such references do not teach building an imaging velocity model with an elastic imaging algorithm taking into account the amplitude and phase characteristics of elastic waves, especially the phase behavior of the elastic waves.
The present disclosure presents a method (and work flow) to build seismic imaging velocity models and to perform seismic imaging by using elastic RTM in a way fully utilizing the elastic scattering information existing in the data.